1. Field of the Invention
This invention relates to improvements in video receiver systems and in particular relates to a modification to the phase locked loop (PLL) circuit of a video tuner/demodulator by which a more viewable picture is obtained from a noisy video signal by reducing bandwidth of the PLL loop response.
2. State of the Prior Art
It is well known that a reduction in the receiver bandwidth ahead of the video demodulator reduces "sparklies" caused by impulse noise present in the video signal being demodulated. Sparklies are flecks superimposed on the picture displayed on a video monitor or television screen, which degrades the quality of the video picture. In areas where the received video signal level is very weak, extraneous impulse noise picked up by the receiving system or produced by the RF amplifier stages of the receiver system may make the video picture unviewable. This is a common condition in fringe television receiving areas.
Video noise reduction (VNR) achieved by bandwidth reduction ahead of the video demodulator results in diminished picture fidelity. This approach to video noise reduction is linear, i.e, picture degradation is proportional to video noise reduction, and can only be carried so far before the picture is degraded too much for viewing. Post demodulation linear filtering of the video signal is also known, but has been found to be even less effective.
One approach to extending the signal to noise (S/N) ratio threshold of an FM video demodulator at which a useful picture is obtained involves the use of a phase locked loop (PLL) demodulator, and is incorporated in commercial receivers, particularly satellite video receivers. One shortcoming of this approach is that PLL demodulators respond in a non-linear manner (all or nothing), i.e. have a large step response, to impulse noise and the PLL momentarily loses lock thereby creating a large black or white spec of noise in the picture, limiting the demodulator's threshold extension capability.
Improvements in this method of video noise reduction are needed.